The switching power supplies to be built in electronic appliances have been required to have a good noise characteristic, a high coupling for yielding a good output voltage characteristic, a low temperature-rise property and a high reliability, in addition to be compact, light in weight and efficient. A new technology has been requested for developing a converter transformer meeting the above requirements.
An example of conventional converter transformer is shown in FIG. 7 and FIG. 8, and other example in FIG. 9 and FIG. 10. FIG. 7 shows a cross sectional view cut into half, FIG. 8 is a connection diagram of windings. FIG. 9 shows a cross sectional view of other exemplary conventional converter transformer cut into half, FIG. 10 is a connection diagram of windings of the converter transformer.
In FIG. 7 and FIG. 8, a converter transformer comprises a inputting winding 1 (hereinafter referred to as primary winding) formed with split windings 1a and 1b for inputting a power supply, and output windings 3, 4, 5, 6, 7 (hereinafter referred to as secondary winding) for supplying electricity to load in the secondary side. The secondary windings 3 and 4 are main secondary windings for supplying major power to load in the secondary side, while the secondary windings 5, 6 and 7 are auxiliary secondary windings for supplying minor power to load in the secondary side. Contained further in the converter transformer includes, an output winding 2 (hereinafter referred to as primary sub-winding) for supplying electricity to a control IC in the primary side, a coil bobbin 8, an insulating material 11 provided between windings, and a magnetic core 10 made of a ferrite core.
A conventional converter transformer of this category is assembled with the split winding 1a of primary winding 1, the secondary windings 3-7, the split winding 1b and the primary sub-winding 2, each wound in the order around coil bobbin 8 with the insulating material 11 of 25 .mu.m or 50 .mu.m thick polyethylene terephtalate sticking tape in between the windings, and the magnetic core 10. The secondary windings 3, 4, 5, 6 and 7 are normally disposed between the split winding 1a and the split winding 1b, and the split winding 1a and the split winding 1b are connected in series.
In the above described conventional structure, a large encountering area may be provided between primary winding 1 and secondary windings 3-7, because each winding can share a large width; furthermore, the encountering distance between primary winding 1 and secondary windings 3-7 can be made very small as the insulation between windings is made with a thin film. As a result, the coupling between primary winding 1 and respective secondary windings 3, 4, 5, 6 and 7 may be raised to quite a high level, bringing about a high conversion efficiency with low temperature rise of a converter transformer.
Drawback with the conventional converter transformer includes that when the major output load, which is being supplied from main secondary windings 3 and 4, is varied the output voltage of auxiliary secondary windings 5, 6 and 7 makes a substantial fluctuation because of the high coupling with primary winding 1.
Further, the position of winding is easily displaced and the coupling is quite high in the conventional constitution, therefore a slight displacement of winding produces a substantial change in coupling, and shift in the output voltage. For example, if a protection circuit for overvoltage detection is contained in the output of primary sub-winding 2 the protection circuit readily makes an erroneous operation, because of the above described change in coupling.
Furthermore, there is a large stray capacitance existing between primary winding 1 and the whole secondary windings 3-7, and the impedance is small; as a result, high frequency noise component may be easily transmitted to, and the noise characteristic is unfavorable.
Now in the following, the other exemplary conventional converter transformer is described referring to FIG. 9 and FIG. 10. An identical portion as in the earlier described conventional example is represented by putting the same symbol in FIG. 9 and FIG. 10. The conventional converter transformer is assembled with a primary winding 1 and secondary windings 3, 4, 5, 6 and 7. The secondary windings 3 and 4 are main secondary winding for supplying to major output load, the secondary windings 5, 6 and 7 are auxiliary secondary winding for supplying to minor output load. Contained further in the converter transformer are a primary sub-winding 2, a coil bobbin 8a, and a magnetic core 10 made of a ferrite core.
A conventional converter transformer of this category is assembled with a primary winding 1 and a primary sub-winding 2 wound around in a winding groove 9b located in substantially the middle among a plurality of winding grooves 9a, 9b, 9c, 9d in coil bobbin 8a; secondary windings 3 and 4 for supplying to major output load, each has been split into 3a, 3b and 4a, 4b and wound around winding groove 9a, 9c, respectively, and connected in parallel; secondary windings 5, 6 and 7 wound around in a winding groove 9d positioned further outside for supplying to minor output load; and a magnetic core 10.
In the above described conventional structure, primary winding 1 and primary sub-winding 2 are wound around in the winding groove 9b located in substantially the middle of coil bobbin 8a; secondary windings 3 and 4 for supplying to major output load, each split into 3a, 3b and 4a, 4b are wound around groove 9a, 9c, respectively, and connected in parallel; and secondary windings 5-7 are wound around in a groove 9d positioned further outside for supplying to minor output load. As a result, the encountering distance between primary winding 1 and secondary windings 5-7 for supplying to minor output is large, and the encountering area is small. Therefore, the coupling may be lowered, and the fluctuation in output voltage of auxiliary secondary windings 5-7 may be suppressed even when the major output load, which is being supplied from main secondary windings 3 and 4, is varied.
Further, because of a structure in which the displacement of winding is hard to occur the shift of coupling is small, so is the output voltage. Therefore, even if, for example, a protection circuit for overvoltage detection is contained in the primary sub-winding 2, erroneous operation of the protection circuit seldom takes place because the voltage fluctuation due to shift in the coupling is small.
Furthermore, as the stray capacitance between primary winding 1 and the whole secondary windings 3-7 is small, the impedance can be made higher, the high frequency noise component is hard to be transmitted to the improvement of noise characteristic.
On the other hand, as the coupling between primary winding 1 and secondary windings 3 and 4 for supplying to major output load is low, the conversion efficiency deteriorates and the temperature rise goes high in the conventional converter transformer.
The present invention provides a small, compact and highly efficient converter transformer that has a good noise characteristic, a high coupling and a good output voltage characteristic, and a suppressed temperature rise as well as a high reliability.